This invention relates to the reduction of SO.sub.x components in the flue gas discharged from the catalyst regenerator of catalytic cracking units, and especially fluidized catalytic cracking (FCC) units. More particularly, this invention relates to injecting into the FCC unit an aluminum-containing compound to reduce the amount of SO.sub.x emitted from the catalyst regenerator.
In the petroleum industry, high boiling hydrocarbon feedstocks are charged to FCC units so that, by contact with a moving bed of catalyst particles, the feedstock is converted to a more valuable hydrocarbon product, such as gasoline, having a lower average molecular weight and a lower average boiling point than said feedstock. The most typical hydrocarbon feedstock so treated in FCC units consists of heavy gas oil, but on occasion such feedstocks as light gas oils, naphtha, reduced crudes, and even whole crudes are also subjected to catalytic cracking to yield low boiling hydrocarbon products.
Catalytic cracking in FCC units is usually accomplished in a cyclic process involving catalytic reaction, steam stripping, and catalyst regeneration. The hydrocarbon feedstock is blended with an appropriate amount of catalyst particles, and the mixture so produced is then passed through a catalytic reactor, commonly called a riser, wherein a catalytic cracking reaction zone is maintained such that at a temperature between about 800.degree. and 1100.degree. F. the feedstock is converted into gaseous, lower boiling hydrocarbons. After these lower boiling hydrocarbons are separated from the catalyst in a suitable separator, such as a cyclone separator, the catalyst, now deactivated with coke deposited upon its surfaces, is passed to a stripper. In the stripper, the deactivated catalyst is contacted with steam so as to convert some of the coke to hydrocarbon product vapors, which are then combined with the vapors received from the cyclone separator and transferred to other facilities for further treatment. Meanwhile, catalyst particles are recovered from the stripper, and because only a small proportion of the coke was removed in the stripper, the catalyst is introduced into a regenerator wherein, by combustion in the presence of an oxygen-containing gas such as air, the remaining larger proportion of the coke is removed and the catalyst reactivated. The cyclic process is then completed by again blending the reactivated catalyst particles with the feedstock entering the FCC unit.
A major difficulty with cracking hydrocarbons in an FCC unit is presented when the hydrocarbon feedstock contains sulfur compounds, usually in the form of organic sulfur compounds. Ideally, the sulfur compounds in the feedstock are converted to H.sub.2 S in the catalytic reaction and stripping zones so that all the contained sulfur in the feedstock is recovered as H.sub.2 S with the low boiling hydrocarbon product vapors, which product vapors are then conveniently desulfurized by absorption of the H.sub.2 S in an alkanolamine solution. But in practice, it has been found that some sulfur compounds remain (or are converted to forms which remain) with the coke on the deactivated catalyst recovered from the stripper. Hence, when the coke is combusted in the regenerator, a flue gas containing SO.sub.x compounds is produced.
The flue gas, if untreated, is a Source of pollution. Although about 90-95% of the sulfur compounds entering an FCC unit with the feedstock are ultimately removed as H.sub.2 S and other gaseous sulfur compounds, the remaining 5-10% left with the coke and converted to SO.sub.x compounds in the regenerator represents a significant environmental and engineering problem. For a typical FCC unit handling a feedstock containing about 1.5 weight percent sulfur components (calculated as elemental sulfur) fed at a rate of about 50,000 barrels per day, the amount of SO.sub.x compounds discharged from the regenerator in one day is between about 10 and 20 tons (calculated as SO.sub.2).
Because of the concern created by the discharge of SO.sub.x compounds in such large quantities, various methods have been devised to reduce SO.sub.x emissions from FCC units to environmentally tolerable levels. Recently, attempts have been made to reduce such SO.sub.x emissions by recycling with the catalyst particles in the FCC unit a metal-containing component, commonly called a "sulfur getter," that reacts in the regenerator with the gaseous SO.sub.x compounds to yield a solid sulfur compound. The produced sulfur compound is then reconverted to the active "sulfur gettering" form by passage through the riser and stripper wherein H.sub.2 S is released from the sulfur compound. The released H.sub.2 S is then removed with the low-boiling hydrocarbons produced in the stripper and riser and separated from said low-boiling hydrocarbons, usually by contact with an alkanolamine solution.
One method illustrating the use of a "sulfur-getter" is described in U.S. Pat. No. 3,835,031, wherein magnesium oxide is incorporated on the catalyst as the "sulfur getter." In the regenerator, the magnesium oxide reacts with the SO.sub.x compounds to produce magnesium sulfate, thereby preventing the release of SO.sub.x compounds from the regenerator. As the catalyst particles are recycled through the catalytic cracking and steam stripping zones maintained in the riser and stripper, respectively, the magnesium sulfate is converted back to magnesium oxide while the contained sulfur is released as hydrogen sulfide and collected with the low boiling hydrocarbon products. Thus, the catalyst particles, when recycled to the regenerator again, contain a magnesium compound (i.e., magnesium oxide) in an active form for removing SO.sub.x.
A similar FCC process is disclosed in U.S. Pat. No. 4,071,436. In this process, solid particles of reactive alumina are introduced into the FCC unit and are recycled with the catalyst particles. Again, as with the process previously described, the alumina particles remove SO.sub.x in the regeneration zone and are reactivated to forms once again active for remoxing SO.sub.x by passage through the riser and stripper.
In yet another process, set forth in U.S. Pat. No. 3,699,037, solid magnesium or calcium hydroxide particles are fed to the FCC unit to react with the SO.sub.x released in the regenerator, thereby producing magnesium sulfate or calcium sulfate and substantially reducing the SO.sub.x concentration of the flue gas discharged from the regenerator.
Although each of the foregoing processes may result in some desulfurization of the flue gas leaving the regenerator, none is completely satisfactory. In some instances, the "sulfur getter" not only reacts with the produced SO.sub.2 but also interferes with the activity of the catalyst for yielding the intended low-boiling hydrocarbon product. In some instances, such as in the process described in U.S. Pat. No. 4,071,436, the "sulfur getter" is only about 0.5-2.0% reactive for removing SO.sub.x compounds, thereby causing inefficiency in the FCC unit by requiring the recycle of a material which is 98% or more inert. Another problem resides in the fact that the "sulfur getter" often proves difficult to reactivate, or, in the case of the process described in U.S. Pat. No. 3,699,037, the "sulfur getter" apparently is not at all capable of being reactivated in the riser and stripper under the usual conditions maintained therein.
Accordingly, it is an object of the invention to provide a process in which a "sulfur getter" is used to effectively remove SO.sub.x compounds from the regenerator flue gas without adversely affecting the activity or selectivity of the cracking catalyst. It is a further object to provide a "sulfur getter" which, once it has reacted with SO.sub.x compounds in the regenerator, is easily regenerated to its active form by passage through the riser and stripper of the FCC unit under the usual conditions maintained therein. It is a further object to provide a "sulfur getter" agent which comprises a relatively high proportion of material active for removing SO.sub.x compounds. It is yet a further object of the invention to increase the effectiveness of the "sulfur getter" by maximizing its degree (or extent) of dispersion upon the catalyst surfaces.